The present invention relates to a demultiplexer for an optical time-division multiplexed (OTDM) digital signal that has a signal wavelength λs and is transmitted with a bit rate B. The invention further relates to a method for demultiplexing such a digital signal.
In a typical OTDM system, an optical pulse source on the transmitter side generates a pulse train with a channel bit rate BC that equals the base rate of electronic data streams fed to the OTDM system. At present, the highest electronic base rate available is 40 Gbit/s. The optical pulse train generated by the pulse source is coupled into n optical branches in which modulators are driven by the electrical data streams. Each modulator imprints the incoming data stream on the pulse train, thereby generating an optical data signal with the channel bit rate BC. The n optical data signals, which represent n different channels, are interleaved by a delay-line multiplexer on a bit-by-bit basis (bit interleaving TDM) or on a packet-by-packet basis (packet interleaving TDM). The multiplexer generates a multiplexed optical data signal with a multiplex bit rate BM=n×BC. The multiplexed signal is then launched into a transmission medium, for example a single-mode optical fiber.
On a receiver side, an optical demultiplexer usually de-interleaves the channels, because electronic devices are not capable of directly processing signals with bit rates BM. The demultiplexed signals with the channel bit rate BC are finally reconverted by optoelectronic devices into electric signals for further processing.
In ultra-high-speed OTDM transmission systems having bit rates of more than 40 Gbit/s, pulse durations are extremely short. For a 160-Gbit/s system, for example, the time slot for a single bit is only 6.25 ps wide. In systems using return to zero (RZ) pulses, i.e. pulses that return to zero power level within each time slot, the width of a pulse is even shorter, namely about one half of the time slot width.
Such extremely short pulse durations pose very high demands on demultiplexers that are one of the key components in OTDM transmission systems. Demultiplexers are not only required for the transmission systems as such but also for bit error rate (BER) measurement equipment that is used for the design and testing of transmission systems. Demultiplexers that are capable of separating pulses in ultra-high bit rate optical transmission systems require very short time windows, a high extinction ratio and a weak polarization dependence.
Until now there are several approaches for realizing such ultra-high bit rate demultiplexers.
One approach is to use an electro-absorption modulator that is capable of producing a short time window with a high extinction ratio when isolating a desired channel from an incoming pulse train. Electro-absorption modulators are semiconductor devices that have an absorber region whose properties can be changed by the switching of an electric field applied across the absorber region. The electric field can be changed fast enough to make switching rates of up to 40 GHz possible.
These devices, however, have intrinsically a high insertion loss, typically of more than 10 dB. This loss has to be compensated by amplifiers that add ASE (amplified spontaneous emission) noise to the signal. Furthermore, due to the intrinsic transfer function of the modulator, there is always a trade-off between a high extinction ratio on the one hand and a short time window on the other hand. Finally, electro-absorption modulators that are used for demultiplexing ultra-high bit rate data signals require numerous costly components that are difficult to manufacture, to package and to optimize.
Another approach for realizing ultra-high bit rate demultiplexers is to use a non-linear optical loop mirror (NOLM) that may be considered as a fiber version of a Sagnac interferometer. A two-by-two directional coupler divides a data pulse train into two pulse trains that counter-propagate around a common loop of fiber. Clock-pulsed intensity variations induce phase modulations within the non-linear optical regime of the fiber as a result of the Kerr effect. These phase modulations alter the phase relationship of the paired counter-propagating pulses. Upon return to the directional coupler, the combined pulses are switched between the input and output of the coupler in accordance with their interference properties. Constructively interfering pulses are reflected back through the coupler's input, whereas destructively interfering pulses are transmitted through the coupler's output.
NOLMs are practically not limited in speed but suffer from an intrinsic instability due to the long span of fibers used in the fiber loop. Moreover, it is difficult to achieve a polarization independence of the device. Finally, demultiplexing by NOLMs require ultra-short optical clock pulses that induce the Kerr effect in the loop.